| Aging Quartz crystal aging applies to the cumulative change in frequency over time, which results in a permanent change in the operating frequency of the crystal unit. The rate of change of frequency is fastest during the first 45 days of operation. Many interrelated factors are involved in aging, some of the most common being: Internal contamination, excessive drive level, surface change of the crystal, various thermal effects, wire fatigue and frictional wear. Proper circuit design incorporating low operating ambients, minimum drive level and static pre - aging will greatly reduce all but the most severe aging problems. A major factor in the aging rate of a quartz crystal is in the method of encapsulation since the sealing of the crystal case leaves contamination and oxygen within the crystal environment. Where crystals are concerned, the 2 most common methods of encapsulation for through hole crystals are resistance weld and cold weld. Typical aging rates are as follows; |
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Drive Level Drive level is the level of power dissipated in the crystal as a result of the operating circuit. Rated or test drive level is the power at which the crystal is specified and any deviation from the rated level will affect the crystal performance: therefore, the actual drive level should reasonably duplicate that specified. AT-Cut crystals can withstand a considerable overdrive without physical damage: however, the electrical parameters are degraded at excessive drive. Low frequency crystals (especially flexural mode crystals) may fracture if overdriven. Drive level ratings range from 5µW below 100kHz to about 10mW in the 1 - 30 MHz region for Fundamental Mode crystals. Overtone crystals which are generally used above 30 MHz are often rated at 1 - 2 mW |
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| Duty Cycle (Symmetry) (Tw/T) Ratio of full and half cycles. For CMOS loading duty is rated at 1/2 VDD, and for TTL loading at |
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| Equivalent Series Capacitance (C1) Energy distortion to the (equivalent) internal charge capacitance component of the crystal resonator, at the series resonant frequency. |
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| Equivalent Series Resistance (ESR) As can be seen from the equivalent circuit of a crystal, there is the parameter R, which is defined as the motional resistance. This value has a direct relationship to the Q-Factor and Activity of the crystal. Activity can also be termed ESR or Equivalent Series Resistance. The lower the ESR values, the higher will be the amplitude of oscillation in the oscillator circuit. The actual value of ESR is somewhat dependent on the load capacity presented to the crystal in use, and can be calculated by the following formula: |
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ESR = R1 ( 1 + Co / CL )2 W |
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It is extremely important to take into account the ESR value when designing a crystal oscillator, as crystals with a high ESR value will be less inclined to begin oscillation than one with a low ESR. However, the actual ESR values for a particular frequency and design can be easily manufactured by the |
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| Frequency (fo) Number of waves (cycles) per second. The relationship between frequency and cycle is: |
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fo (Hertz Hz) = 1 / T (Sec.) 1 kHz = 1ms, 1 MHz = 1µS, 1 GHz = 1nS |
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| Frequency Tolerance Precision (f / f) Under specified conditions at an ambient temperature of 25 °C, the difference in actual (measured) frequency from the nominal frequency. |
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| Frequency Stability (f / fo) Within standard temperature and operational voltage ranges, the drift in the output frequency. The output frequency drift including frequency temperature characteristics and frequency voltage characteristics. Taking the frequency at 25°C as the reference, the change in frequency in response to ambient temperature. |
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| Frequency Voltage Characteristics Taking the output frequency at the central voltage in the operating voltage range as the reference, the change in output frequency to voltage. Causes of this change are changes in crystal deformation, and changes in IC internal constants for IC's mounted in the oscillator and RTC. The effects of the IC's are larger. |
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| Fundamental Mode First harmonic crystal vibration state. The AT resonator frequency is determined by the AT resonator thickness of the crystal, but even with the same thickness the third overtone will be approx. 3 times the frequency of the fundamental. With tuning fork type resonators, the second overtone is about six times the fundamental. |
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| Insulation Resistance (IR) Resistance between leads, or between lead and case package (conductive package). |
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| Load Capacitance (CL) Effective capacitance (series equivalent charge capacitance) of the oscillation circuit as seen from the pins of the crystal oscillator. This capacitance is determined as a condition when the crystal oscillator is connected to the oscillation circuit, and will determine the output frequency. Load capacitance approximation: |
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CL = CG X CD / ( CG + CD ) + CS (CS = stray capacitance) |
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| Maximum Drive Level (GL) Rating for the drive level. Current or power input over this level may result in characteristic degradation or destruction. |
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| Maximum Supply Voltage (VDD - GND) Maximum rated value for power input to the power supply pin. Input over this value may result in characteristic degradation or destruction. |
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| Nominal Frequency (f) Nominal value of frequency of crystal resonator. |
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| OCXO Oven Controlled Crystal Oscillator. Oscillator with additional circuitry and packaging to keep the environment, and thus the temperature range over which the oscillator has to operate, constant. |
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| Operating Temperature Range (Tsol) Temperature range where specification characteristics are fulfilled. |
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| Operating Voltage (VDD) Voltage input to VDD pin which will support continuous operation with specification characteristics. |
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| Origin Frequency (fo) Oscillation source frequency of oscillator inside oscillation system. |
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| Oscillation Circuit Circuit needed to oscillate crystal resonator. Circuit will differ with type of resonator and frequency. |
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| Oscillation Start Time (Tosc) The time from power on until the waveform stabilises. However, voltage rise times depend on the power supply, therefore the time is measured from a specific set of initial conditions. |
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| Oscillator Crystal with additional circuitry that forms the crystal output into a square wave. |
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| Out put Enable (OE) Output is switched to high impedance, and wired OR connection can be used to select multiple outputs (frequency). OE pin - low. Output is high impedance = disabled. Oscillation is not stopped, so the clock after disabled is cleared is not synchronised with OE (clock is continuous). |
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| Output Fall Time (tTHL) The time it takes for the output waveform to change from the high voltage (high level) to the low voltage (low level). Also called waveform fall time. |
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| Output Frequency (fo) The output frequency from the oscillator circuit or the crystal oscillator system. |
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| Output load conditions The types and quantities (power) of the loads that can be connected to the oscillator. |
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Calculated for TTL-1 as: |
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| Output Rise time (tTHL) The time it takes for the output waveform to change from the low voltage (low level) to the high voltage (high level). Also called waveform rise time. Also called waveform rise time. Often specified between 0.4V and 2.4V for TTL or 10% to 90% for CMOS. |
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| Overtone crystals Because of the physical properties and geometry of an AT Cut quartz blank, a crystal can vibrate at many frequencies. The lowest frequency is called the fundamental frequency and can be supplied up to about 45 Mhz. Higher frequencies (to over 300 MHz) are achieved by operating the crystal at odd overtones, 3rd, 5th, 7th, 9th and 11th etc. and tuning the circuit so that the crystal oscillates at its designed overtone frequency. Overtone crystals are specially processed for plane parallelism and surface finish in order to enhance their performance at the required overtone frequency. The overtone frequency is higher than the equivalent harmonic multiple of the fundamental by approximately 25 kHz per overtone. |
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| Pullability The pullability of a crystal refers to a crystal operating in the parallel mode and is a measure of the frequency change as a function of load capacitance. Pullability is important to the circuit designer who wishes to achieve several operating frequencies with a single crystal by means of switching various values of load capacitance. |
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| Recommended Drive Level Excitation level for optimum oscillation characteristics. |
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| Shunt Capacitance (Co) Charge capacitance between the 2 electrodes in the crystal oscillator. |
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| Soldering conditions Soldering conditions that can be assured at mounting. Temperatures or times over these limits may result in characteristic degradation or destruction. |
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| Standby (ST) Function that halts crystal resonator oscillation and frequency division. Cuts the current consumed by the oscillators circuit and the frequency division stage. ST Pin - High or Open : Specified frequency output. ST Pin - Low : Output is high level, clock stops. Because oscillation is halted, there is a delay of maximum 10mS, (0.3mS TEP), before clock output stabilises. If ST is also dropped to low, output is high impedance but output is also unstable after function is restarted for the same reason. |
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| Storage Temperature (Tstg) Maximum absolute rating for the discharged state (no input of voltage, current or power). Exposure to temperature over this level may result in characteristic degradation or destruction. To assure precision, store at room temperature whenever possible. |
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| TCXO Temperature Compensated Crystal Oscillator. Oscillator with additional circuitry that incorporates a feedback loop, where changes in temperature are reflected by changes in voltage, which in turn change the frequency to compensate for the temperature change. |
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| VCXO Voltage Controlled Crystal Oscillator. Oscillator with additional circuitry that allows the frequency to be changed by varying an external voltage. ( See Pullability) |
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